The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device which has a linear characteristic between injected current and optical output and achieves a stable oscillating spectrum, and which is particularly suitable for applications in optical communications.
Semiconductor laser devices are commonly used for applications in optical communications. In particular, InGaAs 980 nm-band semiconductor laser devices are frequently used as the pumping light source for optical amplifiers in optical fiber communication systems.
The semiconductor laser device for use as the pumping light source for the optical amplifier, for example, is required to have a stable, optical output and emission spectrum with respect to the injected current.
Specifically, the current versus light output characteristic is required to be linear in order to increase the reliability of the optical amplification operation, for example. It is also preferable that the emission spectrum is in the stable longitudinal multimode lasing for suppressing the influence of returned light.
Referring to FIG. 4, the structure of a conventional InGaAs 980 nm-band semiconductor laser device will be described. FIG. 4 is a sectional view showing the structure of the conventional InGaAs 980 nm-band semiconductor laser device.
As shown in FIG. 4, the conventional InGaAs 980 nm-band semiconductor laser device 10 has a layered structure including an n-type GaAs substrate 12 having a thickness of 100 xcexcm. On the substrate, an n-type AlGaAs cladding layer 14 having a 2 xcexcm film thickness, an active layer 16 having a quantum-well structure with a pair of InGaAs/GaAs, a p-type AlGaAs cladding layer 18 having a 2 xcexcm film thickness, and a p-type GaAs cap layer 20 having a 0.3 xcexcm film thickness are consecutively grown epitaxially.
Of this layered structure, the p-type cap layer 20 and the upper portion of p-type cladding layer 18 are formed as a stripe-shaped mesa structure having a 4 xcexcm width.
Except for the top of p-type cap layer 20, a passivation film 22 implemented by an SiN film is formed on the side walls of the mesa structure and on the p-type cladding layer 18.
On top of the exposed p-type cap layer 20 and passivation film 22, a p-side electrode 24 including layered metal films of Ti/Pt/Au is formed. On the bottom surface of GaAs substrate 12 is formed an n-side electrode 26 including layered metal films of AuGe/Ni/Au.
Referring to FIGS. 5A to 5C, the process of manufacturing the above-mentioned conventional semiconductor laser device 10 will be described. FIGS. 5A to 5C are sectional views of the substrate during the respective steps of the fabrication of the conventional InGaAs 980 nm-band semiconductor laser device.
First, on the n-type GaAs substrate 12 are epitaxially formed, by MOCVD, the n-type AlGaAs cladding layer 14 having 2 xcexcm film thickness, the active layer 16 with the quantum-well structure of the InGaAs/GaAs pair, the p-type AlGaAs cladding layer 18 having 2 xcexcm film thickness, and the p-type GaAs cap layer 20 having 0.3 xcexcm film thickness, in the recited order. Thus, the layered structure is formed as shown in FIG. 5A.
Then, the p-type cap layer 20 and the upper portion of p-type cladding layer 18 are etched to form the stripe-shaped mesa structure which is 4 xcexcm in width, as shown in FIG. 5B.
After forming the SiN film 22 as the passivation film on the entire top surface of the wafer, the SiN film 22 is etched to expose the cap layer 20 as shown in FIG. 5C.
The entire top surface of the wafer is then covered with the Ti/Pt/Au layered metal films by evaporation, thereby forming the p-side electrode 24. The top surface of GaAs substrate 12 is polished to have a thickness of 100 xcexcm, and thereafter layered metal films of AuGe/Ni/Au are evaporated on the entire top surface of the substrate. Thus, the semiconductor laser device 10 can be fabricated as shown in FIG. 4.
In the above-described conventional semiconductor laser device 10, the band-gap energy Eg1 of n-type GaAs substrate 12 is 1.41 eV and the band-gap energy Eg2 of active layer 16 is 1.27 eV. The relationship Eg1 greater than Eg2 enables the light emitted from the active layer to propagate through the substrate.
If the polished bottom surface of GaAs substrate 12 of semiconductor laser device 10 is mirror-finished, the light that propagated through GaAs substrate 12 is reflected by the bottom surface of the substrate and recombined as reflected light with the light from the active layer, as shown in FIG. 6.
When the reflected light from the bottom surface of the substrate combines with the light from the active layer, there arise the following two problems.
The first problem is that a kink phenomenon appears in the current versus light output characteristic which, as shown in FIG. 7, adversely affects the linearity of the optical output with respect to the injected current. Such a kink phenomenon makes it impossible to maintain a stable APC (automatic power control) operation.
The second problem is that the influence of the returned light becomes large. As the reflected light from the bottom surface of the substrate combines with the light from the active layer, ripples appear in the emission spectrum at about 3 nm intervals, as shown in FIG. 8. This is a phenomenon due to the formation of a hybrid resonator formed by an ordinary Fabry-Pxc3xa9rot resonator and 22a and the substrate.
In such a case, a longitudinal mode is selected at a 3 nm spacing. As the injected current is varied, the selected wavelength shifts mainly by the thermal effect while maintaining the 3 nm mode spacing, thereby varying the output and resulting in a mode hopping noise which is observed as an excessive noise. The lasing mode becomes a single longitudinal mode due to the ripples, so that the tolerance against the returned light deteriorates.
Accordingly, it is an object of the invention to provide a semiconductor laser device in which the optical output and emission spectrum are stable with respect to the injected current.
The semiconductor laser device according to the present invention is directed to a semiconductor laser device in which an active layer having a band-gap energy Eg2 is epitaxially grown on a semiconductor substrate having a band-gap energy Eg1, where Eg1 greater than Eg2, the device being characterized in that an absorption medium layer for absorbing laser light lased by the active layer is formed on the bottom surface of the semiconductor substrate.
The absorption medium layer may be formed in any manner; however, it is preferably formed by an alloying reaction between a metal electrode layer formed on the bottom surface of the semiconductor substrate and the semiconductor substrate because of the easiness of the process. More specifically, if the semiconductor substrate is a GaAs substrate, the absorption medium layer formed on the bottom surface of the semiconductor substrate is an InGaAs layer, which is formed by an alloying reaction between In and GaAs. Namely, the metal electrode layer formed on the bottom surface of the GaAs substrate includes an In layer adjacent to the bottom surface of the substrate, and the absorption medium layer is an InGaAs layer obtained, after the formation of the metal electrode layer on the bottom surface of the substrate, by effecting a thermal processing to alloy the In of the metal electrode layer with the GaAs of the substrate.
Thus, the semiconductor laser device according to the present invention has an absorption medium layer formed on the bottom surface of the substrate that absorbs the laser light lased by the active layer. As a result, although the laser light lased by the active layer passes through the semiconductor substrate from the active layer side of the substrate surface to the bottom surface of the substrate due to the band-gap energy Eg2 of the active layer being smaller than the band-gap energy Eg1 of the semiconductor substrate, the laser light can be absorbed by the absorption medium layer.
Because the amount of laser light reflected by the bottom surface of the semiconductor substrate can thus be reduced in the semiconductor laser device according to the invention, the linearity of the optical output with respect to the injected current can be maintained, the emission spectrum can be stabilized, and the lasing mode is unlikely to assume a single longitudinal mode.